JavaScript buttonの練習

algebra ˈældʒəbrə 代数学

In javascript, the first character must be a letter, or a underscore(_), or a dollersign($). Like in HTML and CSS, class and ID name of the first character. And ofsource, case sensitive.

  var lastname, lastName;
  lastname = "Doe"
  lastName ="John"

  two differrent variables.

button can change the headline

also change paragraph

HTML JavaScript, HTML Button, HTML DOM Button Object

var x=50; var y=100; var z=x + y;

z is...

id=""は一回のみ適用、class=""は複数 getElementByClassName()は、getElementById()よりちょっと難しい？

getElementById() idが複数あってしまったら最初の方に適用される。そもそもidは一回用だから複数あってはダメなのだが。

try it にidを２つ付けた場合。index=0優先。

HTML DOM getElementByClassName() Method

Text

car and cost.

ternary ３つのユニット、要素を持つもの、3進法、3元の、3値

expoential notation "123e5 = 12300000" "123e-5 = 0.00123"

var cars = ["Saab", "Volvo", "BMW""] array index numbers are [0, 1, 2]

Object
variables= {name:"values", name:"values", name:"values",};

var pesron2 = {firstname2:"Ted", lastname2:"Mosby", age2:"50", eycolor2:"blue"};

An empty string has both a legal value and a type. If 'var car= "" '
The value is:(empty/nothing/null)
The type is : string;

JavaScript arithmetic, == equal, === strict equal, != Unequal, !== Strict unequal, ++ icrement, -- decrement, % Modulus(remainder),

JavaScript Assignment

Operator	Example	Same as
=	x = y	x = y
+=	x+=y	x = x + y
-=	x -= y	x= x - y
*=	x *= y	x = x*y
/=	x /= y	x = x/y
%=	x %= y	x = x % y
<<=	x <<= y	x = x << y
>>=	x >>= y	x = x >l;> y
&=	x &= y	x = x & y
^=	x ^= y	x = x ^ y
|=	x |= y	x = x | y
**=	x **= y	x = x ** y

CSS Table

  Difference between undefined and null
  undefined and null are equal in value, but different in type:

  typeof undefined  //undefined
  typeof null       //object

  null === undefined   //false
  null == undefined    //true 

\'= ' , \"= " ,\\ = \

.replace()

Replace /g is replace all values, HD10 and ipad are android OS.

  common mistakes

  var x = 10; //number
  var y = 20; //number
  var z = "The answer is: " + x + y; //string
  //=== "The result is: 1020
  var x = 10 //number
  var y = 20 //number
  var z = "30"; //string
  var result = x + y + z;
  //==== "3030"

  Javascript Numbers
  
  typeof NaN;  // returns "number"

Javascript Type conversationsl, typeof

arrray .length of last index number

declareing: var fruits = ["Banana", "Orange", "Apple", "Mango"]

local, grobal variables
declareing variables inside Javascript function, code can only use the function.

  var "variables name"= "something variables"; grobal

  function myFunction() {
  var carName("variables name") = "Volvo"; this is local variables: code here CAN use carName
}

 //code here can NOT access the carName(local variables)

Array.forEach() calls a function for each array element. That is good, I think.